The Zika Cases in the United States

The Zika virus has been transmitted in the United States for the first time through sexual contact, health officials said.

The case in Dallas County in Texas was confirmed by the Centers for Disease Control and Prevention Tuesday. Until now, the more than 30 cases of the virus reported in the continental U.S. were among travelers who returned to the country from Latin America, which, particularly Brazil, has seen a spike in Zika cases in recent months.

The individual in Texas was infected with the mosquito-borne virus after having sexual contact with a Zika-infected person who had traveled to a country where the virus is present, according to the Dallas County Health and Human Services. The virus is usually transmitted by mosquito bites.

“Now that we know Zika virus can be transmitted through sex, this increases our awareness campaign in educating the public about protecting themselves and others,” said Zachary Thompson, the Dallas health department’s director. “Next to abstinence, condoms are the best prevention method against any sexually-transmitted infections.”

Zika is primarily transmitted through Aedes mosquitoes, the same insects that spread the dengue and Chikungunya viruses. About 1 in 5 people infected with the virus will get sick. The CDC says the disease is usually mild, and those disease and those who do get sick show symptoms—fever, rash, joint pain, and red eyes—for up to a week. But the virus has been linked to a condition called microcephaly in babies born to infected mothers in Latin America and the Caribbean. Infants born with microcephaly have smaller-than-normal head size, which can result in severe cognitive, neurological, and motor disabilities. In Brazil, which saw its first case of Zika last May, there were 20 times more microcephaly cases in 2015 than in 2014.

It is unclear whether after this case of sexual transmission the CDC will designate the continental U.S. an area where Zika is locally transmitted. Cases of the virus have been reported in Puerto Rico, a U.S. territory, and 21 nations, as well as three French and two Dutch territories in the Caribbean, according to the CDC. Transmission by mosquitos has not been reported in the continental U.S.

The World Health Organization on Monday designated the spread of the virus and the increase in cases of microcephaly a public-health emergency of international concern. The classification—the most serious action the organization can take—is intended to galvanize an international response to the virus. It’s rare, too: WHO previously declared such emergencies in the Ebola and polio outbreaks in 2014 and the H1n1, or swine flu, outbreak in 2009.

Some governments in Latin America advised women to hold off on having children as the outbreak grows. Brazil and Colombia have suggested women avoid getting pregnant for several months, while El Salvador has asked them to wait until 2018.

Health officials have advised the public to use insect repellant, protect skin with long-sleeved shirts and pants, remove standing water—the breeding grounds for mosquitos—in and around their homes, and avoid being outside at dusk and dawn, when the bugs feed.

We want to hear what you think about this article. Submit a letter to the editor or write to letters@theatlantic.com.